Big bang nucleosynthesis refers to the process of element production during the early phases of the universe,
shortly after the
Big Bang. It is believed to be responsible for the formation of hydrogen, its isotopedeuterium, helium in its varieties 3He and 4He,
and the isotope of lithium7Li. Hydrogen nuclei
(protons) are believed to have formed as soon as the
temperature had dropped enough to make the existence of
free quarks impossible, but for a while the number
of protons and neutrons was almost the same, until the
temperature dropped enough to make its slight mass difference favor the protons. Isolated neutrons are not stable, so the ones that survived are the ones that could
bond with protons to form deuterium, helium, and lithium.

Why didn't all the neutrons bond with protons and made
all elements up to iron? While the temperature was dropping,
the universe was also expanding, and the chances of collision were getting smaller. Also very important is the
fact that there is no stable nucleus with 8 nucleons. So there was a bottleneck in the nucleosynthesis that
stopped the process there. In stars, the bottleneck is
passed by triple collisions of 4He nuclei (the triple-alpha process), but in the expanding early universe, by the time there was enough 4He the density of the universe had dropped too much to make triple collisions possible.

Using the Big Bang model, it is possible to make predictions about elemental
abundances and to explain some observations which would otherwise be difficult
to account for. One such observation is the existence of deuterium. Deuterium
is easily destroyed by stars, and there is no known natural process which would
produce significant amounts of deuterium. Another observation is the existence
of far more helium in old stars that can be accounted for by stellar nucleosynthesis.

The relative abundances of the different elements produced are dependent on the number of photons per baryon. As the number
of photons is dominated by the cosmic microwave background radiation, measuring the primordial abundances of those
elements allow us to know the density of baryons, that is, of matter in the universe.

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